1. Field of the Invention
The present invention relates to a safety device for preventing a battery exploding and a method for manufacturing the safety device, thereby also preventing related damage to surrounding equipment, the safety device being in the form of a thin safety valve welded or otherwise attached to a battery casing to seal a gas vent in the casing so as to allow the release of gas that has risen to a certain pressure in the battery casing, such as a casing of a lithium battery or hydrogen-nickel battery.
2. Discussion of the Background
Explosion prevention safety devices for batteries include those such as the device disclosed by JP-A-59-79965. In that device, the battery cover is in the form of a thin strip of rolled stainless steel 0.4 mm thick having a circular groove 0.2 mm wide with a base 0.2 mm thick formed by pressing, to comprise a safety valve. A rise in internal pressure in the battery causes rupturing along the circular groove, blowing off the safety valve and allowing the gas to be released.
However, based on the rolling specification the rolled stainless steel strip has a tolerance of .+-.3 to 5 .mu.m. This means that a groove formed to have a width of 0.2 mm and a floor thickness of 0.02 mm in the strip having a thickness of 0.4 mm.+-.3 to 5 .mu.m will have a floor thickness of 0.2 mm.+-.3 to 5 .mu.m, and the pressure at which the safety valve will blow when the thickness of the floor is 0.02 mm+5 .mu.m will not be the same as the pressure at which the safety valve will blow when the thickness of the floor is 0.02 mm-5 .mu.m. Moreover, the precision of the thickness produced by pressing will vary slightly depending on the precision of the die and how long it has been in use, the variation being around .+-.5 to 7 .mu.m which, when included with respect to the thickness of the groove floor, further increases the variation in internal battery pressure at which the safety valve will blow. Furthermore, the rolled steel material always includes impurities, and if such impurities should form in the floor thickness of the groove, it can weaken that portion to the point where a rise in internal pressure in the battery can result in that portion giving way and allowing the release of the gas, even if the pressure is not high enough to require such release. In extreme cases, such impurities at that portion can form pinholes, resulting in a defective product.
Another arrangement comprises coating the stainless steel strip with a photoresist layer, overlaying the photoresist layer with a photomask that defines the shape of the safety valve groove, exposing and developing the photoresist, removing the unexposed photoresist portion defining the shape of the safety valve groove to expose that portion of the stainless steel strip, etching the exposed portion of the stainless steel strip to reduce the thickness of the floor of the safety valve groove, and then removing the remainder of the photoresist layer from the stainless steel strip. However, the material used is still a rolled stainless steel strip, so there are still the problems of the .+-.3 to 5 .mu.m thickness tolerance and the existence of impurities, in addition to which the precision of the thickness of the floor of the groove formed by etching is .+-.10 .mu.m, worse than the .+-.5 to 7 .mu.m precision of the thickness when the groove is press formed, so that the variation in the internal pressure at which the safety valve blows is even greater than when the groove is press formed.
The basic technical point of the floor of the groove of the safety valve of a battery casing is to prevent an explosion by rupturing when the gas pressure in the casing reaches a set pressure, thereby allowing the gas to escape from the casing. Consequently, the narrower the range of the set pressure, the more reliable the safety device that can be provided. Realizing such a safety device, along with execution by the photolithography method, is an urgent task.